Generally, a hydraulically driven vehicle such as a wheeled hydraulic excavator obtains a desired traveling speed by operating a travel pedal so as to increase or decrease the rotation speed of a prime mover for controlling a pump discharge amount. Among this type of hydraulically driven vehicles, there is a device known in the related art that increases the maximum rotation speed of a prime mover by a predetermined amount to increase the output horsepower when the traveling load is at a predetermined value or more and the travel pedal is operated (see Patent Literature 1).    Patent Literature 1: Japanese Patent Laid-open Publication No. 2001-295681.